a single right or left angle for the turns, the robotcan be commanded to traverse in straight linesand then make the turns at intermediate points inthe course. At each turning point, the steeringmotor drives the wheels to the hard point leftor right; the drive wheels then turn a pre-programmed number of revolutions or until acompass tells the microcontroller that so manydegrees have been turned. At this point, thewheels are turned back to forward and the robotcontinues to the next turning point. Both thegradual steering to make curved turns or thepoint-to-point style of steering can be veryaccurate, but will require a bit of programmingexpertise.

FIGURE 6. LEGO robot using caster wheel.

Differential Steering

Differential or ‘tank style’ steering is the most
popular for experimental robots, especially with
R/C robots as the steering can be done with a
joystick on the R/C transmitter. Most builders of
these types of robot rely on two side wheels with
one caster or an omni-wheel in the front or back
(or maybe one or more at each end). Figure 6
from the SRS’ website shows a simple LEGO robot
with a single caster wheel.

Steering is accomplished by driving the fixed
side drive wheels at different speeds or directions.
An example would be when the right wheel is
slightly faster than the left, the robot makes a
slight turn to the left. If the left wheel is not
turning at all and the right wheel is moving, the
robot makes a sharp turn to the left. In an
extreme situation, if the right wheel is turning
forward and the left wheel is turning backwards,
the robot spins on its axis counter-clockwise.

Robot designers using both a fore and aft
wheel must make sure that the center drive
wheels are not high sided when traversing a
concave surface. One or both casters can be
spring-loaded enough to keep the robot from
wobbling but not too much to prevent the drive
wheels from touching the ground in a slight dip
or hole.

Differential Skid Steering
Uses More Power

FIGURE 7.

Heavy-duty
omni-wheel from
Omniwheel.com.

78 SERVO 04.2010

Differential steering is also referred to as
‘skid’ steering. Mobile robots with differential
steering have no problem with wheel friction if
they use one or more swivel casters at one or both
ends of the robot and only a single pair of drive
wheels. If the robot designer tries to use two or
more pairs of parallel drive wheels that only face
in the fore-aft position, then skidding occurs when
the robot is steered — just like when tank or
bulldozer tracks skid on turning. An omni-wheel
(as shown in Figure 7) has been used in many
robot designs and is especially applicable when
trying to minimize skid friction. Six-wheel
differential drive robot platforms can use two
standard drive wheels for the center wheel pair,
and two other omni-wheel pairs for the fore and
aft wheels. These wheels can also be powered,
but can allow low friction side skidding in turns.

Skidding creates friction which equates to
power loss. This type of skidding is not a problem